Methods, devices and systems of supporting discontinuous operation in communication systems using vectoring

ABSTRACT

Methods, devices and techniques are disclosed where vectoring is adapted to lines becoming inactive and active, for example in a discontinued operation. In some embodiments, the vectoring is modified based on already present coefficients.

REFERENCE TO RELATED APPLICATION

This application is a continuation of U.S. application Ser. No.14/890,173 filed on Nov. 10, 2015, which claims priority toPCT/EP2015/059736, filed on May 13, 2014, which claims priority to U.S.Provisional Application Ser. No. 61/822,478, filed on May 13, 2013, thecontents of which are incorporated by reference in their entirety.

FIELD

The present application relates to methods, devices and systems relatingto discontinuous operation in communication systems using vectoring.

BACKGROUND

Digital Subscriber Line (DSL) technology, like ADSL, ADSL2, VDSL, VDSL2etc. up to the upcoming G.fast, during all its history, attempted toincrease the bit rate in the aim to deliver more broadband services tothe customer. Unfortunately, copper loops deployed from a Central Office(CO) to customer premises (CPE) are rather long and do not allowtransmission of data with bit rates more than few Mb/s. To increase thebit rates, modern access networks use street cabinets, MDU-cabinets, andsimilar arrangements, generically also referred to as distributionpoints (DP) in this application: cabinets are connected to the CO by ahigh-speed backbone communication line, like multi-gigabit passiveoptical network (GPON) and installed close to the customer premises.From these cabinets or other DPs, high-speed DSL systems, such asVery-High-Bit-Rate DSL (VDSL), can be deployed. Current VDSL systems(ITU-T Recommendation G.993.2) have range of operation about 1 km,providing bit rates in the range of tens of Mb/s. To increase the bitrate of VDSL systems deployed from the cabinet, recent ITU-TRecommendation G.993.5 defined vectored transmission that allowsincreasing bit rates up to 100 Mb/s per direction.

Recent trends in the access communications market show that 100 Mb/s isstill not sufficient and bit rates up to 1.0 Gb/s are required. This,based on current techniques, could be only achieved if copper pairsconnecting the CPE as short as 50-100 m. Operation using so short loopsrequires installation of many small street/MDU cabinets calledDistribution Points (DP) that intends to serve a very small number ofcustomers.

Vectoring is used in systems operating from a DP, to reduce far-endcrosstalk (FEXT), which is helpful for obtaining high bit rates. Toperform vectoring, transmissions from all lines deployed from the DP aresynchronized (i.e., all downstream transmission are aligned in time andall upstream transmissions are aligned in time). Further, in thedownstream direction, vectoring is implemented by precoding the transmitsignals (at the DP), so that each signal includes transmission of itsown data and FEXT cancellation signals intended to compensate FEXTgenerated by other lines. In the upstream direction, the receivedsignals from all lines are jointly processed at the DP to remove FEXTcomponents (also referred to as crosstalk cancellation or equalization).Details of the described vectoring technique are defined e.g. in G.993.2for VDSL2, but may be applicable to other communication protocols.

Operation using short loops requires installation of many small DPs thatintend to serve a small number of customers, such as 8-16, as shown byrecent analysis. In some cases, a higher number of served users, like 24or even 32 may be present. Therefore, it would be helpful if DPs allowedvery flexible installation practices: they should be light and easy toinstall on a pole or house wall, or basement, without air-conditioning.The most challenging issue for these flexible connection plans isproviding DPs with power. The solution intended so far is so-called“reverse feeding” when the equipment of the DP is fed by the connectedcustomer.

Small internal space of the DP and use of reverse power feeding mayimply substantial restrictions on the power consumption of a DP.Therefore, special means to reduce power consumption have been developedand new ones are on development. This application mainly concerns oneefficient method of power savings, called Discontinuous operation.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a communication system according to some embodiments.

FIG. 2 is a diagram illustrating discontinuous operation.

FIG. 3 illustrates transmission on a line using two sets of precodermatrices.

FIG. 4 illustrates a precoder of a single distribution point portaccording to an embodiment.

FIGS. 5A and 5B illustrate a simulation of signal-to-noise ratios for a16 line binder with equally distributed lengths. In FIG. 5A, all linesare active, in FIG. 5B, one line is disabled.

FIG. 6 illustrates behavior of the example of FIG. 5 after an update ofmatrix coefficients.

FIGS. 7A and 7B show signal-to-noise ratios for a 16 line binder withequally distributed lengths from 50 meters to 250 meters with nonlinearprecoding. In FIG. 7A, all lines are active, in FIG. 7B one line isdisabled.

FIG. 8 illustrates a downstream system model with a Tomlinson-Harashimaprecoder.

FIGS. 9A and 9B illustrate signal-to-noise ratio for a 16-line binderwith equally distributed length from 50 meters to 250 meters withnonlinear precoding. In FIG. 9A, all lines are active, in FIG. 9B, twolines are disabled. The channel matrix has been updated, and disabledlines were the last ones added to the precoder.

FIG. 10 illustrates a receiver of a single distribution point portaccording to an embodiment.

FIG. 11 is a diagram illustrating precoder settings and updates in adownstream direction.

Embodiments will be described in the following in detail with referenceto the attached drawings. It should be noted that these embodimentsserve as illustrative examples only and are not to be construed aslimiting. For example, while embodiments may be described havingnumerous details, features or elements, in other embodiments some ofthese details, features or elements may be omitted and/or may bereplaced by alternative features or elements. In other embodiments,additionally or alternatively further features, details or elementsapart from the ones explicitly described may be provided.

Communication connections discussed in the following may be directconnections or indirect connections, i.e. connections with or withoutadditional intervening elements, as long as the general function of theconnection, for example to transmit a certain kind of signal, ispreserved. Connections may be wireless connections or wire-basedconnections unless noted otherwise.

In some embodiments, efficient possibilities for updating vectoringcoefficients, for example precoding coefficients, in systems usingdiscontinuous operation may be discussed. For example, a precoder matrixmay be updated based on actually active lines.

In other embodiments, other techniques may be employed.

Turning now to the figures, in FIG. 1 a communication system accordingto an embodiment is shown. The system of FIG. 1 comprises a providerequipment 10 communicating with a plurality of CPE units 14-16. Whilethree CPE units 14-16 are shown in FIG. 1, this serves merely as anexample, and any number of CPE units may be provided. Provider equipment10 may be central office equipment, equipment in a distribution point(DP), or any other equipment used on a provider side. In case providerequipment 10 is part of a distribution point, it may for example receiveand send data from and to a network via a fiber optic connection 110. Inother embodiments, other kinds of connections may be used.

In the embodiment of FIG. 1, provider equipment 10 comprises a pluralityof transceivers 11-13 to communicate with CPE units 14-16 via respectivecommunication connections 17-19. Communication connections 17-19 may forexample be copper lines, e.g. twisted pairs of copper lines.Communication via communication connections 17-19 may be a communicationbased on a multicarrier modulation like discrete multitone modulation(DMT) and/or orthogonal frequency division multiplexing (OFDM), forexample an xDSL communication like ADSL, VDSL, VDSL2, G.Fast etc., i.e.a communication where data is modulated on a plurality of carriers, alsoreferred to as tones. In some embodiments, the communication system mayuse vectoring, as indicated by a block 111 (e.g. a crosstalk reductioncircuit like a precoder or equalizer) in FIG. 1. Vectoring comprisesjoint processing of signals to be sent and/or received to reducecrosstalk.

A communication direction from provider equipment 10 to CPE units 14-16will also be referred to as downstream direction, and a communicationdirection from CPE units 14-16 will be also be referred to as upstreamdirection. Vectoring in the downstream direction is also referred to ascrosstalk precompensation, whereas vectoring in the upstream directionis also referred to as crosstalk cancellation or equalization.

Provider equipment 10 and/or CPE units 14-16 may include furthercommunication circuits (not shown) conventionally employed incommunication systems, for example circuitry for modulating, bitloading, Fourier transformation etc.

In some embodiments, communication via communication connections 17-19is a frame-based communication. A plurality of frames may form asuperframe. In some embodiments, the communication uses time divisionduplexing, as will be explained later.

Besides others methods, discontinuous operation, which is employed insome embodiments, is one of the ways to save power, e.g. in DSL lines.With Time Division Duplexing (TDD) transmissions in opposite directionsalternate: the unit of transmission time, called TDD frame, is dividedinto two parts called Downstream Transmission Opportunity (DS_TO) andUpstream Transmission Opportunity (US_TO). The DS_TO is a time periodwhen downstream transmission occurs, and US_TO is the time period whenupstream transmission occurs. With discontinuous operation, when no datais available to be transmitted during the TO or a part of the TO, thetransmitted simply stops transmission instead of filling up the time gapby idle symbols, as it is done in some conventional systems (framepadding).

With discontinuous operation the number of symbols transmitted in aparticular line during a particular TDD frame may be different, which isillustrated in FIG. 2. FIG. 2 also shows markers that indicate when theactive transmission will expire. Alternatively, the duration of thetransmission in a number of consecutive TDD frames may be set in advanceand indicated to the CPE during management communication (whichindicates the number of transmitted symbols in a TDD frame). In thiscase Idle symbols (padding) will be added to frames in which the numberof symbols required to transmit available data is less than thecommunicated value (shown in the right-hand side of FIG. 2 with dashedlines). It should be noted that markers and Idle symbols in embodimentsare not supposed to be used at the same time).

Use of discontinuous operation in embodiments is beneficial for powersavings, especially for services that have dynamically changing numberof active users and bit rates. For ultra-high bit rate systems deployedfrom the DP, further below techniques will be presented to facilitatediscontinuous operation with vectoring.

Discontinuous operation was previously proposed and defined. It providesan ability to transmit symbols only when there is user data availablefor transmission, while powers the line off when no data fortransmission is available. Therefore, with discontinuous operation, thenumber of symbols transmitted over a particular line of a vectored groupduring a particular TDD frame is different. In this sense, if vectoringis used, the number of lines in the vectored binder is changing on asymbol-per-symbol basis and pre-coding/post-coding associated withvectoring may need to be updated on a symbol-per-symbol basis, which isequivalent of changing the set of lines of the vectored group everysymbol. This very frequent update of the precoder in all lines inconventional approaches would require super-fast processing capabilitiesor big extra memory to store pre-computed precoder coefficients—bothmethods results in high power consumption and extra complexity.

Another conventional approach, illustrated in FIG. 3, uses a secondmethod, and in the aim to reduce the number of sets of coefficients tobe stored, it proposes to group lines together, apply padding (tominimize the number of groups) and update precoding matrices smallnumber of times (1-2) during the TDD frame (see FIG. 3).

In FIG. 3 there are two groups of lines, and padding symbols are addedto establish small number of groups (e.g. 2-3) with same number oftransmit symbols in each group. Idle symbols (padding) are transmittedwith same power as regular data symbols, which reduces power savings insuch an approach. In case the bandwidth of a particular line needs to beincreased or substantially reduced, re-grouping is needed. The precodermatrices for each group may be stored and kept during the groupassignment. This may limit the flexibility of such a conventionalsystem, while padding itself may substantially reduce potential powersavings.

In embodiments, a specific way of updating the precoder matrices (in thedownstream direction) and decoder (postcoder) matrixes (in the upstreamdirection) are proposed, so that only few coefficients need to beupdated and special order in the updating process allows almost no extrapower and complexity. This way some embodiments avoid powerfulcomputations (and high power consumption) and change the matrices onsymbol bases without even re-computing matrix coefficients. This, insome embodiments, essentially provides a full flexibility in picking thenumber of transmit symbols in every TDD frame. The method according tosome embodiments is reliable, guarantees quality of service (QoS), andprovides efficient power saving at both DP and CPE.

Next, downstream vectored operation will be discussed.

A functional model of downstream vectoring according to some embodimentsis presented in FIG. 4 (LD stands for Line Driver, AFE for Analog FrontEnd and IFFT for Inverse Fast Fourier Transform). A vectoring precoderis included into the transmitter to compensate FEXT (Far-End Crosstalk):one input of the precoder is the actual transmit signal for the line attone i, Z(i), others are inputs are from other lines of the vectoredgroup and intended for FEXT cancelling. The precoder applies inputs fromother lines via precoding coefficients and special processing tocompensate FEXT generated by other lines into line i.

Two types of precoders are conventionally employed: linear andnonlinear. With linear precoder, FEXT cancellation signals are added tothe transmit signal Z(i) after multiplication on selected complexcoefficients that represent FEXT coupling in the binder. With anonlinear precoding, a nonlinear operation, such as MOD, is used toimprove vectoring performance against noise.

If discontinuous operation is used, each line transmits different numberof symbols in each TDD frame. That means that from a certain symbol(time instant T₂ for Line 2 in FIG. 2) the number of actual vectoredlines changes—it may change after every symbol. When a line stopstransmission (e.g., when Line 2 in FIG. 2 stops transmission at timeT₂), this causes change in the FEXT generated into other lines, and theprecoder coefficients (coefficients of the precoding matrix) in allother lines need to be updated to accommodate this change. Thus,precoding matrix may need to be updated every symbol in some cases.

The complexity and processing associated with the update may depend onthe type of the precoding. In the worst case, precoding matrices of allremaining lines need to be obtained and stored at the transmitter priorthe session, which requires big amount of memory to keep all thematrices associated with all possible symbol combinations. This doesn'tseem to be practical. Thus, the above-described conventional approachuses line grouping and padding, as shown in FIG. 3, to leveragecomplexity increase.

It should be noted that switching of the lines ON and OFF duringdiscontinuous operation should not involve substantial changes in theimpedance of the line; if the impedance of the transmitter or receiverchanges substantially, it may results in substantial changes of FEXTcoupling between all lines in the binder (especially at highfrequencies). This may require re-estimating the channel prior to thecoefficient update, which is impossible to be done on symbol-by-symbolbases. Therefore, in the following description it is assumed nosubstantial impedance changes associated with ON/OFF duringdiscontinuous operation, which assumption is at least in goodapproximation applicable to many practical situations.

If the impedance stays unchanged, removal of one or more lines from thevectored group may still cause very substantial performance degradationif the matrices of the remaining active lines are not updated. Theparticular value of the degradation depends also on the type ofprecoding.

An example in FIG. 5 shows a simulation example for an SNR degradationin lines of a 16-line binder using linear precoding, caused by a removalof just one line—the SNR reduction in some lines is more than 10 dB,which is obviously unacceptable (FIG. 5A shows the SNR with all linesactive, FIG. 5B the SNR with one line switched off.)

To improve the SNR again, an update of matrix coefficients is necessary.After the update of the relevant matrix coefficients, the SNR gets back,as it is shown in FIG. 6.

For the same 16-line binder using nonlinear precoding(Tomlinson-Harashima vectored precoding, as proposed in someconventional approaches), the change of SNR after one line of the groupis disabled is much more dramatic. The SNR degradation is shown in FIGS.7A and 7B, FIG. 7A showing SNR before disabling of one line and FIG. 7Bshowing SNR after the disabling.

Next, discontinuous operation with linear precoder according toembodiments will be discussed.

Some embodiments are a method or a device having discontinuous operationand using linear precoder, which results in minimized number ofcomputations for coefficient updates and thus can be performed on symbolbases in some embodiments.

With linear precoding, a received signal û can be presented as:û=H·P·u  (1)

where u is the transmit signal prior to precoding, H is a channelmatrix, P is the precoder matrix. The frequency equalized (FEQ) matrix Gis considered as a part of the channel matrix H to simplify equations.

If some lines are disabled in the aim to save power, the correspondingrows and columns from the channel matrix H and the precoder matrix P areset to zero.

Accordingly, we partition the matrix into matrixes of active (a) and ofdisabled (d) lines. Before deactivation of the line, for perfectcrosstalk precompensation the following holds.

$\begin{matrix}{{\begin{bmatrix}H_{aa} & H_{ad} \\H_{da} & H_{dd}\end{bmatrix} \cdot \begin{bmatrix}P_{aa} & P_{ad} \\P_{da} & P_{dd}\end{bmatrix}} = \begin{bmatrix}I & 0 \\0 & I\end{bmatrix}} & (2)\end{matrix}$

I is the unitary matrix with non-zero elements only in the maindiagonal. With disabled lines (due to discontinuous operation), equation(2) must still be fulfilled, which requires a different matrix P′_(aa).H _(aa) ·P′ _(aa) =I  (3)

Based on the matrix inversion lemma, equation (3) is found for thecalculation of the matrix coefficients for active lines:P′ _(aa) =P _(aa) −P _(ad) ·P _(dd) ⁻¹ ·P _(da).  (4)

With this simple coefficient re-computation, the matrix can be reducedto keep orthogonality after some lines have been disabled. For the casethat only one line is disabled and the diagonal elements of P are equalto 1, the equation (4) simplifies to:P′ _(aa) =P _(aa) −p _(da) ·p _(ad) ^(T)·1⁻¹  (5)

In case that the method is used for discontinuous operation, linedeactivation is temporary and therefore a recomputation of thecoefficients is disadvantageous. The operation of equation (5) can be,however, incooperated into the precoding operation (i.e., can beimplemented by changing the order of multiplication and summation duringprecoding). This doesn't require additional computation resources.

For all active lines, we havex=P·u  (6)

x being the transmit signal(s) after precoding. The transmit signal fora single line is given by equation (6):x _(i) =p _(i) ^(T) ·u  (7)

With one line deactivated, we get:x _(a) =P _(aa) u _(a) −p _(da) ·p _(ad) ^(T) ·u _(a)  (8)

Accordingly, for the remaining active lines, in case when one line isdeactivated (discontinued), we have:x _(ai) =p _(ia) ^(T) ·u _(a) −p _(di) ·p _(ad) ^(T) ·u _(a)  (9)

which uses the same precoder coefficients as when all lines are active.

Equation (9) shows that computation of the precoding output has the samenumber of multiply-accumulated (MAC operations) as when all lines areactive since the value p_(ad)·u_(a) needs to be computed only once forall lines and needs to be scaled for each line differently by thecorresponding coefficient p_(id).

If more than one line is discontinued, equation (9) still holds becausethis can be seen as sequential deactivation of a number of single lines.Thus, when more than one line is deactivated, the number of MACoperations for the computation on the active lines is still the same.

An alternative to the sequential calculation is the inversion of thematrix P_(dd) according to equation (10).x _(a) =P _(aa) u _(a) −P _(da) ·P _(dd) ⁻¹ ·P _(ad) ·u _(a)  (10)

To avoid recalculation of the coefficients for P_(dd) ⁻¹, the firstorder approximation of matrix inversion according to equation (11) canbe used.

$\begin{matrix}{\lbrack P_{dd}^{- 1} \rbrack_{ij} \approx \{ \begin{matrix}{- p_{ddij}} & {{{for}\mspace{14mu} i} \neq j} \\p_{ddij} & {{{for}\mspace{14mu} i} = j}\end{matrix} } & (11)\end{matrix}$

With more than one line disabled, the assumption of 1 on the diagonal(result of initial PSD normalization) does no longer hold. Thus, thismethod may results in changes of the transmit PSDs (on the precoderoutput). Some lines transmit more power than before, while otherstransmit less. However, these PSD changes are rather small (˜2-4 dB),and if lines are enable and disabled for rather short time and randomly,as takes place in discontinuous operation, the average spectrum does notincrease.

Next, discontinous operation with nonlinear precoder will be discussed.

The model of Tomlinson-Harashima nonlinear precoder is presented in FIG.8 with a forward matrix P_(f), a feedback matrix P_(b) and an equalizerdiagonal matrix G. n represents noise.

The nonlinear precoder matrices (P_(b) and P_(f)) are given by equations(12) to (15) shown below. The coefficients are calculated based on theQR decomposition according to equation (12), where Q is a unitary matrixand R is an upper triangular matrix. The matrix Π is a permutationmatrix defining the encoding order of the data streams.Q·R=(Π·H·Π ^(T))^(T)  (12)P _(f)=Π^(T) ·Q ^(x)  (13)P _(b) =I−diag(R)⁻¹ ·R ^(T)  (14)G=Π ^(T) diag(R)⁻¹Π  (15)

For discontinuous operation, in embodiments, the encoding order isselected such that the line that is disabled first is encoded first. Bydoing so, it is guaranteed that if the input signal of the disabled lineis set to zero, a corresponding modulo output signal u_(mod) is alsoequal zero and the nonlinear operation does not change.

So, if lines are discontinued in the order that is reversed to the orderthey were added to the precoding matrix, in embodiments, the impact onSNR of the remaining lines is much less dramatic than shown in FIG. 6(right-hand side). The technique is possible if the duration oftransmission over every line is known prior the start of the TDD frame(i.e., if matrix is computed prior time T₀ in FIG. 1 with known valuesof T₁, T₂, . . . T_(N)).

However, in some cases there may still be an impact on performance. Thisis because the signal u_(mod) at the output of the MOD (e.g. modulo)operation is transformed by the matrix P_(f) and must be zero to avoiddistortions caused by deactivation of the corresponding lines.

Therefore, some embodiments comprise replacing the signals of the lineswhich were transmitted prior to deactivation, which, in embodiments, isgiven by equation (16).P′ _(f aa) =P _(f aa) +H _(aa) ⁻¹ ·H _(ad) ·P _(f da)  (16)

In case that the precoder matrix P_(f) shall not be changed, theoperation according to equation (16) can be performed to calculate thetransmit signal for the remaining active lines. In contrast to thelinear precoder case, additional coefficients for the matrix (H_(aa)⁻¹·H_(ad)) are required.x _(a) =P _(f aa) ·u _(mod a)+(H _(aa) ⁻¹ ·H _(ad))·P _(f da) ·u_(mod a)  (17)

This modification of the precoder matrix results in a change of thetransmit PSD (Power Spectral Density), similarly as in the linearprecoder case. Similarly as in case of linear precoder, this inembodiments is not expected to be harmful because the discontinued linesreduce the total power transmitted over the binder. After this update ofthe P_(f), in an example scenario, the SNR got improved and recoveredback to the values achieved before the discontinuous operation.

An example is shown in FIGS. 9A and 9B, FIG. 9A illustrating a casewhere all lines are active, and FIG. 9B illustrating a case where twolines were disabled and techniques as disclosed herein for updatingcoefficients where applied.

Next, upstream vectored operation will be discussed.

The functional model of the receiver (e.g. in a DP) is presented in FIG.10. The output of the decoder is a composition of the signal receivedfrom the line and the sum of signals received from all other activelines multiplied by matrix coefficients, which corresponds to theaforementioned crosstalk cancellation or equalization.

If a receiver of a particular line is switched off, the correspondinginput of the decoder is zeroed and FEXT components carried by the lineare zeroed too. Therefore, similarly to the case of the downstream, thereceiver shall update decoder matrix coefficients upon the number ofactually transmitting lines changes. This is rather similar to the caseof downstream. However, such coefficient recalculations, while greatlyimproving SNR related to FEXT, may, in some cases, result in an increaseof background noise in some lines. At least for most relevant cases,this is expected to be insignificant (in fact, for most relevant casesthe background noise will decrease).

Next, update of decoder matrix coefficients for crosstalk cancellationor equalization will be discussed.

For the following analysis we assume that when discontinued operation isused, the input of the decoder from the disabled line will be zero (thereceiver is switched off, however in the way that the impedance of theline doesn't change (to avoid changes in FEXT coupling).

The coefficients of the updated matrix in some embodiments can be foundin the way that is similar to the downstream:

Upstream model similar to equation (1):û=G·H·u  (18)

Here, the equalizer matrix G has non-zero nondiagonal elements and isused for crosstalk cancellation. Coefficient correction of the decodersimilar to equations (4, 18):G′ _(aa) =G _(aa) −G _(ad) ·G _(dd) ⁻¹ ·G _(da)  (19)

Correction of the receive signal instead of coefficient recalculationaccording to equation (19):û _(i) =G _(ai) y _(a) −G _(ds) ·G _(dd) ⁻¹ ·G _(ad) ·y _(a)  (20)

This changes the noise environment, because the receive signal yconsists of received signal plus noise: H·u+n.

This noise increase can be compensated by temporary increase of thetransmit PSD, or by bit loading change. For both cases additionalprotocol may be required in some embodiments.

In some embodiments, to avoid updating of the decoder matrixcoefficients described above, the receiver is kept operating, so thatFEXT signals accumulated by the line are added to the decoder. Thisrequires AFE, FFT and precoder to be left ON. Same as in the downstreamdirection, the dynamic range of the AFE may be reduced if used frequencyspectrum is limited to 100 MHz (none or very few pairs has close tonegative or negative impulse-to-SNR ratio). This gives additional powersaving.

Next, a fast power allocation update according to some embodiments willbe described.

To avoid PSD increase in downstream and noise increase in upstream, afast update of the transmit subcarrier gains can be used in someembodiments.

In the downstream direction, the transmit gains scale the input of theprecoder such that the output of the precoder does not violate transmitPSD. The scale factor update for each configuration of inactive lines ispre-computed by the DP for the following TDD frames and communicated tothe CPEs.

For the upstream direction, the CPEs receive the update of thesubcarrier gains table for the configurations contained in the followingTDD frames. For frequencies up to 100 MHz where the direct channel canbe assumed to be the strongest path between transmitter and receiver,this scaling will increase performance on average.

In one embodiment that involves a linear precoder, the transceiver atthe DP computes the precoder matrix and normalizes the PSD at aninitialization of the vectored group. Further, for each symbol positionin each TDD frame, the DP updates the precoder matrix based on actuallyactive lines using any method as described above. Similarly, the DPcomputes the decoder matrix during the initialization and updates it onsymbol-to-symbol bases using any method as described above.

The DP may adjust transmit PSD normalization if one or more lines areturned to a long term deactivation. Bit loading adjustment in thedownstream direction may be a part of this procedure.

In another embodiment that involves a linear precoder, the DPre-computes precoder coefficients and updates PSD normalization at thestart of each TDD frame or a group of same TDD frames (TDD superframe),FIG. 10, taking in account all active lines for a particular TDD frame(lines 1-3 and 5 in Frame N of FIG. 11) or for a particular group of TDDframes. For the example in FIG. 10, the DP performs 3 updates of theprecoder matrix based on actually active lines using the method definedin section 5.1.2. The upstream decoder matrix may be updated onsymbol-to-symbol bases using any method as described above.

The PSD normalization may be performed in the range that allows to keepthe same bit loading; otherwise bit loading may need to be adjusted forthe TDD frame or group of TDD frames.

In another embodiment that involves a nonlinear precoder, the DPre-computes precoder coefficients at the at the start of each TDD frameor each group of same TDD frames (superframe) in the order lines arediscontinuous during the TDD frame. For the scenario in FIG. 11, for TDDframe N lines may, e.g., be precoded in order: #5, #2, #1, #3, and forTDD frame (N+1) they will be precoded in order: #6, #5, #2, #1, #3. Ateach update the DP also modifies appropriately the feed-forward matrixP_(f) using any method defined above.

The PSD normalization may be performed in the range that allows to keepthe same bit loading; otherwise bit loading may need to be adjusted.This is usually only possible for a group of same TDD frames, so the PSDnormalization can be pre-computed and bit loading changes may beexchanged with the CPE.

In another embodiment, to update the precoder for a group of same TDDframes, padding is added to some transmissions of some frames. In allthe mentioned embodiments no action from the receiver is required exceptwhen bit loading or subcarrier gains are modified. Method of exchangingnew values of bit loading and gains may be trivial and is beyond thescope of this invention.

Embodiments introduce special precoding update techniques (methods andalgorithms) and a protocol associated with these updates. The proposedembodiments may serve as a part of the new G.fast standard.

In some embodiments, a vectored transmission correction network thatemploys discontinuous operation is provided, wherein a method or deviceupdates precoder matrices and decoder or postcoder matrices such that asubset of coefficients needs to be updated.

The above described embodiments serve only as examples, and othertechniques may be used as well.

What is claimed is:
 1. A device arranged to transmit data in adiscontinuous operation network, wherein the device comprising: aprocessor, and a memory; wherein the device configured to: transmit onany of the plurality of lines symbols corresponding to the data aretransmitted during a transmission opportunity of a time division duplex(TDD) frame; transmit on any one of the plurality of lines in adiscontinuous operation mode quiet or idle symbols when no data isavailable for transmission on that one of the plurality of lines;perform crosstalk reduction of a plurality of lines in a vectored groupusing a crosstalk reduction circuit; and modify the crosstalk reductionbased on a previous crosstalk reduction in case of a change ofcomposition of the vectored group in the discontinuous operation mode;wherein, for a case where one line of the lines is deactivated, thedevice is configured to perform crosstalk reduction by precompensatingfor crosstalk according to:x _(ai) =p _(ia) ^(T) ·u _(a) −p _(di) ·p _(ad) ^(T) ·u _(a) whereinx_(ai) is a signal sent on active line a, P is a precompensation matrix,i indicates an inactive line and u is a vector of data signals to besent on active lines.
 2. The device of claim 1, wherein the device isconfigured to perform the crosstalk reduction by precompensatingcrosstalk prior to transmitting signals via the lines of the vectoredgroup.
 3. The device of claim 1, wherein the device is configured toperform the crosstalk reduction by canceling crosstalk after receivingsignals from the lines of the vectored group.
 4. The device of claim 1,wherein the device is configured to perform the crosstalk reduction in acase, where only a subset of lines of the lines are active.
 5. Thedevice of claim 4, wherein the crosstalk reduction is based on crosstalkreduction coefficients for a case where all of the lines are active. 6.The device of claim 1, wherein the device is arranged as a distributionpoint.
 7. A device arranged to transmit data in a discontinuousoperation network, wherein the device comprising: a processor, and amemory; wherein the device configured to: transmit on any of theplurality of lines symbols corresponding to the data are transmittedduring a transmission opportunity of a time division duplex (TDD) frame;transmit on any one of the plurality of lines in a discontinuousoperation mode quiet or idle symbols when no data is available fortransmission on that one of the plurality of lines; perform crosstalkreduction of a plurality of lines in a vectored group using a crosstalkreduction circuit; and modify the crosstalk reduction based on aprevious crosstalk reduction in case of a change of composition of thevectored group in the discontinuous operation mode; wherein, for a casewhere one line of the lines is deactivated, the device is configured toperform crosstalk reduction by precompensating for crosstalk accordingtox _(a) =P _(aa) u _(a) −P _(da) ·P _(dd) ⁻¹ ·P _(ad) ·u _(a) wherein formatrix inversion an approximation according to$\lbrack P_{dd}^{- 1} \rbrack_{ij} \approx \{ \begin{matrix}{- p_{ddij}} & {{{for}\mspace{14mu} i} \neq j} \\p_{ddij} & {{{for}\mspace{14mu} i} = j}\end{matrix} $ is used, where x is a vector of signals sent onactive lines, P is a precompensation matrix, and u is a vector of datasignals to be sent on the active lines.
 8. The device of claim 7,wherein the device is configured to perform the crosstalk reduction byprecompensating crosstalk prior to transmitting signals via the lines ofthe vectored group.
 9. The device of claim 7, wherein the device isconfigured to perform the crosstalk reduction by canceling crosstalkafter receiving signals from the lines of the vectored group.
 10. Thedevice of claim 7, wherein the device is configured to perform thecrosstalk reduction in a case, where only a subset of lines of the linesare active.
 11. The device of claim 10, wherein the crosstalk reductionis based on crosstalk reduction coefficients for a case where all of thelines are active.
 12. The device of claim 7, wherein the device isarranged as a distribution point.
 13. A device arranged to transmit datain a discontinuous operation network, wherein the device comprising: aprocessor, and a memory; wherein the device configured to: transmit onany of the plurality of lines symbols corresponding to the data aretransmitted during a transmission opportunity of a time division duplex(TDD) frame; transmit on any one of the plurality of lines in adiscontinuous operation mode quiet or idle symbols when no data isavailable for transmission on that one of the plurality of lines;perform crosstalk reduction of a plurality of lines in a vectored groupusing a crosstalk reduction circuit; and modify the crosstalk reductionbased on a previous crosstalk reduction in case of a change ofcomposition of the vectored group in the discontinuous operation mode;wherein, for a case when one line of the lines is deactivated, thedevice is configured to perform crosstalk reduction by precompensatingcrosstalk using a non-linear precoding according to:x _(a) =P _(f aa) ·u _(mod a)+(H _(aa) ⁻¹ ·H _(ad))·P _(f da) ·u_(mod a) wherein H is a channel matrix, where x is a vector of signalssent on active lines, P is a precompensation matrix, and u_(a) is avector of data signals to be sent on the active lines.
 14. The device ofclaim 13, wherein the device is configured to perform the crosstalkreduction by precompensating crosstalk prior to transmitting signals viathe lines of the vectored group.
 15. The device of claim 13, wherein thedevice is configured to perform the crosstalk reduction by cancelingcrosstalk after receiving signals from the lines of the vectored group.16. The device of claim 13, wherein the device is configured to performthe crosstalk reduction in a case, where only a subset of lines of thelines are active.
 17. The device of claim 16, wherein the crosstalkreduction is based on crosstalk reduction coefficients for a case whereall of the lines are active.
 18. The device of claim 13, wherein thedevice is arranged as a distribution point.
 19. A device arranged totransmit data in a discontinuous operation network, wherein the devicecomprising: a processor, and a memory; wherein the device configured to:transmit on any of the plurality of lines symbols corresponding to thedata are transmitted during a transmission opportunity of a timedivision duplex (TDD) frame; transmit on any one of the plurality oflines in a discontinuous operation mode quiet or idle symbols when nodata is available for transmission on that one of the plurality oflines; perform crosstalk reduction of a plurality of lines in a vectoredgroup using a crosstalk reduction circuit; and modify the crosstalkreduction based on a previous crosstalk reduction in case of a change ofcomposition of the vectored group in the discontinuous operation mode;wherein, for a case where one line of the lines is deactivated, thedevice is configured to perform crosstalk reduction according to:û _(a) =G _(aa) y _(a) −G _(da) ·G _(dd) ⁻¹ ·G _(ad) ·y _(a), whereinû_(a) is a corrected received signal, y_(a) is a received and G is anequalization matrix for a case when all lines are active.
 20. The deviceof claim 19, wherein the device is configured to perform the crosstalkreduction by precompensating crosstalk prior to transmitting signals viathe lines of the vectored group.
 21. The device of claim 19, wherein thedevice is configured to perform the crosstalk reduction by cancelingcrosstalk after receiving signals from the lines of the vectored group.22. The device of claim 19, wherein the device is configured to performthe crosstalk reduction in a case, where only a subset of lines of thelines are active.
 23. The device of claim 22, wherein the crosstalkreduction is based on crosstalk reduction coefficients for a case whereall of the lines are active.
 24. The device of claim 19, wherein thedevice is arranged as a distribution point.